Infectious bacterial plasmids have been found in a wide variety of organisms and frequently are transferable from one species of microorganism to another. The genes of these plasmids may be classified into three types: those required for replication; those required for transfer; those required for neither of the above but which give the plasmid some of its prominent phenotypic characteristics such as drug resistance or toxin production. It is our wish to increase the biochemical understanding of plasmids by studying the regulation of transcription of plasmid genes in DNA-directed cell-free systems for RNA and protein synthesis. Cell-free systems of this sort can be used to analyze those factors responsible for modulating gene expression. Once purified the properties of such factors can be studied in isolation.